class Vertex(object):

    def __init__(self, index):
        self.index = index
        self.neighbors = set()

    def edgelist(self):
        return [set((self.index, n)) for n in self.neighbors]

    def __str__(self):
        return "{}".format(", ".join(map(str, self.neighbors)))


class GraphRegister(object):

    def __init__(self, n):
        self.vertices = [Vertex(i) for i in xrange(n)]

    def add_edge(self, v1, v2):
        self.vertices[v1].neighbors.add(v2)
        self.vertices[v2].neighbors.add(v1)

    def del_edge(self, v1, v2):
        self.vertices[v1].neighbors.remove(v2)
        self.vertices[v2].neighbors.remove(v1)

    def toggle_edge(self, v1, v2):
        if v2 in self.vertices[v1].neighbors:
            self.del_edge(v1, v2)
        else:
            self.add_edge(v1, v2)

    def edgelist(self):
       return map(tuple, frozenset(frozenset((v.index, n)) 
           for v in self.vertices 
           for n in v.neighbors))

    def __str__(self, ):
        return "\n".join(str(v) for v in self.vertices if len(v.neighbors) > 0)

if __name__ == '__main__':
    g = GraphRegister(10)
    g.toggle_edge(0, 1)
    print g